1. Field of the Invention
This invention relates to a planar magnetron sputtering apparatus. More particularly, this invention relates to the improvements in a structure to generate a magnetic field over a target surface. The invention is intended to make an erosion of the target surface uniform and also to deposit the sputtering material uniformly on a substrate.
2. Description of the Prior Art
A sputtering process to coat semiconductor surface with a conductive or insulating material is well known.
When an electric field of direct or alternating voltage is applied in proximity to a target plate in an inert gas of a reduced pressure, it generates gas ions. Gas ions cause particles of target material to be dislodged and to be deposited on a substrate. This method utilizing only an electric field shows relatively uniform deposition on the substrate, however it has a low rate of deposition. Therefore it has not been suitable to form a thick layer on the substrate.
Recently to improve the deposition rate, a magnetron sputtering method has been introduced, wherein an arched magnetic field is superimposed on the electric field. The magnetic flux used herein exits from and returns to the surface of the sputtering target. The conventional magnetic source of the prior art, provided behind the target, comprises inner and outer magnets and a connecting yoke for the opposite sides of both magnets. The magnetic flux, exiting from one pole and returning to the opposite pole, crosses the target twice, thus forms an arch of magnetic field.
Typical magnet arrangements are shown in FIG. 1 and FIG. 2. FIG. 1 shows a cross sectional view of the magnet arrangement 5 and the target 4 provided thereabove, and other parts are omitted for simplicity. This type has a coaxial arrangement of the inner and outer magnet 1 and 2 respectively and a disk-shaped yoke 3. FIG. 2 has a perspective view of another type of magnet, wherein the target is removed. The outer magnet 2 is a box-like one, and the inner magnet 1 is a straight bar type and arranged in the center as shown in FIG. 2.
An electric field is applied normal to the surface of the target, therefore the electrons generated move under the influences of such crossed electric and magnetic fields, ionizing gas molecules and thereby producing a plasma. The motion of electrons is limited in a doughnut shaped region over the target under the arch of magnetic field, and this region may be compared with a tunnel of the arches of magnetic field. By such motion of electrons, a chance to ionize gas molecule is greatly enhanced, as a result, high density plasma is generated. Gas ions are attracted by the electric field to the target surface and erode the target material, thus a correspondingly high rate of deposition is obtained.
The above magnet structures are disclosed in U.S. Pat. No. 3,878,085, issued to J. F. Corbani on Apr. 15, 1975.
One drawback of the above planar magnetic source is a fact that the erosion takes place in a relatively narrow ring-shaped region corresponding to the tunnel width of the magnetic field. The path of an electron leaving the target is approximately perpendicular to the surface thereof, then the parallel component of the arched magnetic field with respect to the target surface, deflects the electron movement along the path of the magnetic tunnel. In the region just above both poles of magnetic source, the magnetic field is almost perpendicular to the target surface, resulting in very small parallel component, therefore the electrons can easily escape from the magnetic tunnel, as a result, the ionization region is limited to a narrow path along the tunnel. This results in low sputtering rates and poor uniformity of deposition on the substrate.
Because the erosion pattern of the target takes a form of an annular valley, the target has to be replaced even when the other area is remaining almost unchanged with very slight sputtering. A life of the target, which generally is expensive, is limited by the progress of erosion in the annular valley area, and the efforts have been made to increase the area of uniform erosion on the target.
Many types of the magnetic source have been proposed to improve an erosion uniformity of the target. Some of them are referred to as follows.
In U.S. Pat. No. 3,956,093, issued to P. S. McLeod on May 11, 1976, a magnetic source having an additional source of variable magnetic field is described.
In U.S. Pat. No. 4,162,954, issued to C. F. Morrison on July 31. 1979, a magnetic source of stacked or rolled magnetic tape to form a solid and flat coil parallel to the target is illustrated.
In U.S. Pat. No. 4,282,083, issued to G. Kertesz, and G. Vago on Aug. 4, 1981, improvements for a Penning sputter source, which results in an increase of the active zone of the target and uniform utilization thereof, are described.
In provisional publication of Japanese Patent, TOKUKAISHYO 58-87270, issued on May 25, 1983, by K. Abe, a magnetic source having a plurality of electro-magnets coaxially arranged is described.
All these patents relates to the structure of a fixed magnetic source with respect to the target. Therefore they can not completely solve the problem, and there still remains an uneven erosion of the target forming a valley or a groove on the target surface.
In order to solve the problem more effectively, a method to move the magnetic source in parallel to the target surface is proposed. A structure, wherein a circular magnet assembly being rotated in parallel with the target and eccentrically around the shaft located at the center of the target, is representative. An example of such a structure is mentioned in U.S. Pat. No. 4,444,643, issued to C. B. Garrett on Apr. 24, 1984.
FIG. 3 shows a schematic cross sectional view of a planar magnetron sputtering apparatus. A magnetic source 10 is provided outside a vacuum chamber 12 and can be rotated eccentrically around a shaft 11 (rotation mechanism is not shown). A target plate 13 of aluminum or aluminum alloy material, for example, is fixed on a backing plate 14 by soldering material 15. A substrate 16 is supported by a holding means 17 and held facing target 13 in parallel therewith. A vacuum chamber consists of chamber wall 18, backing plate 14, insulating plate 19, and two vacuum packing rings 20. During operation of the apparatus, the target is usually heated to a high temperature by ion bombardments, therefore the backing plate 14 is often cooled by the circulating water through the holes 25, or the magnetic source 10 is encased in a jacket 26 (shown by the dashed lines in the figure), which is fixed to backing plate 14 and is cooled by the circulating water.
The chamber is exhausted from exhaust pipe 21, and an inert gas such as argon is introduced from inlet pipe 22. The pressure of the chamber is maintained at 10.sup.-2 to 10.sup.-3 Torr during operation. The negative terminal of a power supply 23 is connected to the backing plate 14, which holds the target 13. An electric field is formed between an anode 24 and the target 13.
In FIGS. 4(a) and 4(b), a plan view of the magnetic source 10, and a cross sectional view along a line A-A are shown. The magnetic source 10 has a coaxial structure having an N-type center pole 30 and an S-type annular pole 31. Each magnet may be consisted of single magnet or a plurality of small magnets.
The magnetic source 10 is rotated around the shaft 11. In FIG. 4(a), the axis of rotation is shown as Or. In FIG. 4(b), target position is partly shown by dashed line 13', and magnetic field is shown by dashed curves 32 with an arrow, which show arched curves above the target surface.
The cross hatched area 33 shows a plasma abundant region, and this annular region, which is correspondingly shown as a ring section 34 in FIG. 4 (a), sweeps the surface of the target having Or as a revolving center. When annular ring 34 rotates around Or, eroding the target surface, a portion of the ring region in a vicinity of point B sweeps much faster than that of point A. The erosion of the target is proportional to the exposed time of the target for the plasma, and, in other words, is proportional to the quantity of the total length of an arc length divided by sweep velocity. The sweep velocity is in this case proportional to the radius of rotation and the total arc length corresponding to each point A, B, and C is shown as dashed curve length X, Y, and Z+Z' respectively in FIG. 4 (a).
The cross sectional view of the resultant erosion pattern is shown in FIG. 5. In FIG. 5, the points A, B, and C correspond to the regions of the same reference characters in FIG. 4(a). As can be seen in the figure, the depth of the groove at region A is deeper than that of the depth at region B and C. The pattern is far from uniform erosion, which is desirable to be as wide as possible.